[es6] Implement Date.prototype[@@toPrimitive] as C++ builtin.

[es6] Implement Date.prototype[@@toPrimitive] as C++ builtin.

This way we don't need to expose JSReceiver::OrdinaryToPrimitive
as runtime function, and we don't need the separate JS trampoline.

This also adds tests for ToPrimitive on date objects, which are
special.

R=mstarzinger@chromium.org
BUG=v8:4307
LOG=n

Review URL: https://codereview.chromium.org/1324713002

Cr-Commit-Position: refs/heads/master@{#30473}